The present invention relates to a semiconductor device comprising a nitride compound semiconductor layer and a method of fabricating the same.
A nitride compound semiconductor such as GaN, InN, or AlN is a material suitable for use in a blue semiconductor laser device or in a transistor operable at a high speed even at a high temperature.
There has conventionally been known a technique for the crystal growth of a nitride compound semiconductor on a Si (silicon) substrate (A. Watanabe et al., Journal of Crystal Growth volume 128 (1993) pp. 391-396).
As a first conventional embodiment, a laser diode comprising a nitride compound semiconductor layer formed on a silicon substrate will be described with reference to FIG. 6.
As shown in FIG. 6, an AlN layer 11 as a buffer layer, a GaN layer 12 as a first contact layer, a first clad layer 13 made of n-type AlGaN, an active layer 14 made of undoped GaInN, a second clad layer 15 made of p-type AlGaN, and a second contact layer 16 made of p-type GaN are stacked successively in layers on a silicon substrate 10. The AlN layer 11 is formed by growing an AlN crystal on the silicon substrate 10. The GaN layer 12 is formed by growing a GaN crystal on the AlN layer 11 at a temperature of 1050xc2x0 C. and doped with an impurity such as Si, Ge, or Se to have the n-type conductivity. To form the GaN layer 12, metal organic vapor phase epitaxial growth (hereinafter referred to as MOVPE) is used.
A p-type electrode 18 made of an Nixe2x80x94Au alloy on the second contact layer 16 with a current restricting layer 17 having an opening of 17a interposed therebetween, while an n-type electrode 19 is formed on a back surface of the silicon substrate 10.
In the first conventional embodiment, a tensile strain is applied from the silicon substrate 10 to the GaN layer 12 and an internal stress is produced in the GaN layer 12 in response to the tensile strain when the temperature of the silicon substrate 10 is lowered from the crystal growth temperature of 1050xc2x0 C. to a room temperature after the formation of the GaN layer 12. This is because the thermal expansion coefficient (2.55xc3x9710xe2x88x926/K) of Si is lower than the thermal expansion coefficient (5.59xc3x9710xe2x88x926/K) of GaN. The internal stress produced in the GaN layer 12 increases disadvantageously to form a crack (crevice) in the GaN layer 12. Thus, the method for the crystal growth of a nitride compound semiconductor on a silicon substrate is not practical.
Therefore, a technique for the crystal growth of a nitride compound semiconductor on a sapphire substrate has been used instead (U.S. Pat. No. 5,777,350).
As a second conventional embodiment, a laser diode comprising a nitride compound semiconductor layer formed on a sapphire substrate will be described with reference to FIG. 7.
As shown in FIG. 7, an AlN layer 21 as a buffer layer, a GaN layer 22 as a first contact layer, a first clad layer 23 made of n-type AlGaN, an active layer 24 made of undoped GaInN, a second clad layer 25 made of p-type AlGaN, and a second contact layer 26 made of p-type GaN are stacked successively in layers on a sapphire substrate 20. The AlN layer 21 is formed by growing an AlN crystal on the sapphire substrate 20. The GaN layer 22 is formed by growing a GaN crystal on the AlN layer 21 by using MOVPE at a temperature of 1050xc2x0 C. The GaN layer 22 is doped with an impurity such as Si, Ge, or Se to have the n-type conductivity. It is to be noted that a device structure composed of the GaN layer 22 as the first contact layer, the first clad layer 23, the active layer 24, the second clad layer 25, and the second contact layer 26 has been partially removed by dry etching till the GaN layer 22 is etched halfway.
A p-type electrode 28 made of an Nixe2x80x94Au alloy is formed on the second contact layer 26 with a current restricting layer 27 having an opening 27a interposed therebetween, while an n-type electrode 29 made of an Nixe2x80x94Au alloy is formed in a space corresponding to the etched portion of the GaN layer 22, i.e., the first contact layer.
According to the second conventional embodiment, a crack is less likely to occur in the GaN layer 22 than in the first conventional embodiment since the difference between the thermal expansion coefficient (7.5xc3x9710xe2x88x926/K) of sapphire (Al2O3) and the thermal expansion coefficient of GaN is smaller than the difference between the thermal expansion coefficient of Si and that of GaN.
In the second conventional embodiment, however, a compression strain is applied from the sapphire substrate 20 to the GaN layer 22 and an internal stress is produced in the GaN layer 22 in response to the compression strain when the temperature of the sapphire substrate 20 is lowered from the crystal growth temperature of 1050xc2x0 C. to a room temperature after the formation of the GaN layer 22, since the thermal expansion coefficient of sapphire is higher than that of GaN. This prevents an improvement in the crystalline characteristics of the GaN layer 22 and causes the first problem that it is difficult to reduce an operating current for the laser diode.
The second conventional embodiment also presents the second problem that it is difficult to fabricate a laser diode having a smooth reflecting mirror surface since it is difficult to cleave the sapphire substrate 20.
To solve the second problem, there has been proposed a technique for forming a semiconductor substrate composed of a thick-film nitride compound semiconductor layer which has been formed by the crystal growth of a nitride compound semiconductor on a sapphire substrate and separated therefrom (Japanese Unexamined Patent Publication No. HEI 7-165498).
As a third conventional embodiment, a method of forming a laser diode by using a semiconductor substrate composed of a nitride compound semiconductor layer will be described with reference to FIGS. 8(a) to (d).
First, as shown in FIG. 8(a), an AlN layer 31 as a buffer layer is formed by growing an AlN crystal on a sapphire substrate 30.
Next, as shown in FIG. 8(b), a GaN layer 32 as a compound semiconductor layer is formed by growing a GaN crystal on the AlN layer 31 at a temperature of 1050xc2x0 C.
Next, as shown in FIG. 8(c), the AlN layer 31 and the GaN layer 32 are separated from the sapphire substrate 30 by removing the sapphire substrate 30 by polishing, whereby a semiconductor substrate 33 composed of the AlN layer 31 and the GaN layer 32 is formed.
Next, as shown in FIG. 8(d), a first contact layer 34 made of n-type GaN, a first clad layer 35 made of n-type AlGaN, an active layer 36 made of undoped GaInN, a second clad layer 37 made of p-type AlGaN, and a second contact layer 38 made of p-type GaN are formed successively on the semiconductor substrate 33. Thereafter, a p-type electrode is formed on the second contact layer 38 with a current restricting layer interposed therebetween and an n-type electrode is formed on a back surface of the semiconductor substrate 33, though they are not depicted, whereby the laser diode is completed. According to the third conventional embodiment, a laser diode having a smooth reflecting mirror surface can be fabricated since the semiconductor substrate 33 is cleaved easily.
However, the third conventional embodiment has the problem that the crystalline characteristics of the GaN layer 32 composing the semiconductor substrate 33 cannot be improved due to the difference between the thermal expansion coefficient of sapphire and that of GaN, similarly to the second conventional embodiment. The third conventional embodiment also has the problem that the crystalline characteristics of the GaN layer are further degraded as the thickness of the GaN layer 32, i.e., the thickness of the semiconductor substrate 33 is increased.
In view of the foregoing, it is therefore an object of the present invention to improve the crystalline characteristics of a nitride compound semiconductor layer composing a semiconductor device.
To attain the object, a first semiconductor device according to the present invention comprises: a substrate having a first thermal expansion coefficient T1; a strain reducing layer having a second thermal expansion coefficient T2, the strain reducing layer being formed on the substrate; and a semiconductor layer having a third thermal expansion coefficient T3, the semiconductor layer being formed on the strain reducing layer and made of a nitride compound represented by AlyGa1xe2x88x92yxe2x88x92zInzN (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), the second thermal expansion coefficient T2 being lower than the first thermal expansion coefficient T1, the third thermal expansion coefficient T3 being lower than the first thermal expansion coefficient T1 and higher than the second thermal expansion coefficient T2.
In the first semiconductor device, the strain reducing layer having the second thermal expansion coefficient T2 lower than the first thermal expansion coefficient T1 is formed on the substrate having the first thermal expansion coefficient T1 and the nitride compound semiconductor layer having the third thermal expansion coefficient T3 lower than the first thermal expansion coefficient T1 and higher than the second thermal expansion coefficient T2 is formed on the strain reducing layer. Accordingly, when the temperature of the substrate is lowered from a temperature for the crystal growth of the nitride compound semiconductor layer to a room temperature after the formation of the nitride compound semiconductor layer, a compression strain applied from the substrate to the strain reducing layer and a tensile strain applied from the strain reducing layer to the nitride compound semiconductor layer cancel out each other. In other words, since the nitride compound semiconductor layer having the third thermal expansion coefficient T3 is formed on a multilayer structure composed of the substrate having the first thermal expansion coefficient T1 higher than the third thermal expansion coefficient T3 and the strain reducing layer having the second thermal expansion coefficient T2 lower than the third thermal expansion coefficient T3, the difference between a mean value of the thermal expansion coefficients of the multilayer structure and the third thermal expansion coefficient T3 can be made smaller than the difference between the first and third thermal expansion coefficients T1 and T3. As a result, an internal stress produced in the nitride compound semiconductor layer in response to the compression strain from the substrate is reduced and cracks are less likely to be formed in the nitride semiconductor layer, so that the crystalline characteristics of the nitride compound semiconductor layer are improved.
In the first semiconductor device, a ratio T1/T3 of the first thermal expansion coefficient T1 to the third thermal expansion coefficient T3 is preferably lower than a ratio T3/T2 of the third thermal expansion coefficient T3 to the second thermal expansion coefficient T2 and the substrate is preferably larger in thickness than the strain reducing layer.
The arrangement further reduces the difference between the mean value of the thermal expansion coefficients of the multilayer structure composed of the substrate and the strain reducing layer and the third thermal expansion coefficient T3, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
In the first semiconductor device, a ratio T1/T3 of the first thermal expansion coefficient T1 to the third thermal expansion coefficient T3 is preferably higher than a ratio T3/T2 of the third thermal expansion coefficient T3 to the second thermal expansion coefficient T2 and the substrate is preferably smaller in thickness than the strain reducing layer.
The arrangement further reduces the difference between the mean value of the thermal expansion coefficients of the multilayer structure composed of the substrate and the strain reducing layer and the third thermal expansion coefficient T3, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
In the first semiconductor device, the substrate is preferably made of sapphire and the strain reducing layer is preferably made of silicon.
In the arrangement, the strain reducing layer having a specified growth interface is formed on the substrate, so that the crystalline characteristics of the nitride compound semiconductor layer formed on the strain reducing layer are further improved.
In this case, the substrate preferably has a main surface having a (0001) plane and the strain reducing layer preferably has a growth interface having a (111) plane.
In the arrangement, the nitride compound semiconductor layer having a growth interface having a (0001) plane is formed on the strain reducing layer formed to have a growth interface having a (111) plane, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
A second semiconductor device according to the present invention comprises: a substrate having a first thermal expansion coefficient T1; a strain reducing layer having a second thermal expansion coefficient T2, the strain reducing layer being formed on the substrate; and a semiconductor layer having a third thermal expansion coefficient T3, the semiconductor layer being formed on the strain reducing layer and made of a nitride compound represented by AlyGa1xe2x88x92yxe2x88x92zInzN (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), the second thermal expansion coefficient T2 being higher than the first thermal expansion coefficient T1, the third thermal expansion coefficient T3 being higher than the first thermal expansion coefficient T1 and lower than the second thermal expansion coefficient T2.
In the second semiconductor device, the strain reducing layer having the second thermal expansion coefficient T2 higher than the first thermal expansion coefficient T1 is formed on the substrate having the first thermal expansion coefficient T1 and the nitride compound semiconductor layer having the third thermal expansion coefficient T3 higher than the first thermal expansion coefficient T1 and lower than the second thermal expansion coefficient T2 is formed on the strain reducing layer. Accordingly, when the temperature of the substrate is lowered from a temperature for the crystal growth of the nitride compound semiconductor layer to a room temperature after the formation of the nitride compound semiconductor layer, a tensile strain applied from the substrate to the strain reducing layer and a compression strain applied from the strain reducing layer to the nitride compound semiconductor layer cancel out each other. In other words, since the nitride compound semiconductor layer having the third thermal expansion coefficient T3 is formed on a multilayer structure composed of the substrate having the first thermal expansion coefficient T1 lower than the third thermal expansion coefficient T3 and the strain reducing layer having the second thermal expansion coefficient T2 higher than the third thermal expansion coefficient T3, the difference between a mean value of the thermal expansion coefficients of the multilayer structure and the third thermal expansion coefficient T3 can be made smaller than the difference between the first and third thermal expansion coefficients T1 and T3. As a result, an internal stress produced in the nitride compound semiconductor layer in response to the tensile stress from the substrate is reduced and cracks are less likely to be formed in the nitride semiconductor layer, so that the crystalline characteristics of the nitride compound semiconductor layer are improved.
In the second semiconductor device, a ratio T3/T1 of the third thermal expansion coefficient T3 to the first thermal expansion coefficient T1 is preferably lower than a ratio T2/T3 of the second thermal expansion coefficient T2 to the third thermal expansion coefficient T3 and the substrate is preferably larger in thickness than the strain reducing layer.
The arrangement further reduces the difference between the mean value of the thermal expansion coefficients of the multilayer structure composed of the substrate and the strain reducing layer and the third thermal expansion coefficient T3, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
In the second semiconductor device, a ratio T3/T1 of the third thermal expansion coefficient T3 to the first thermal expansion coefficient T1 is preferably higher than a ratio T2/T3 of the second thermal expansion coefficient T2 to the third thermal expansion coefficient T3 and the substrate is preferably smaller in thickness than the strain reducing layer.
The arrangement further reduces the difference between the mean value of the thermal expansion coefficients of the multilayer structure composed of the substrate and the strain reducing layer and the third thermal expansion coefficient T3, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
A first method of fabricating a semiconductor device according to the present invention comprises the steps of: forming a strain reducing layer having a second thermal expansion coefficient T2 on a substrate having a first thermal expansion coefficient T1; and forming a semiconductor layer having a third thermal expansion coefficient T3 on the strain reducing layer, the semiconductor layer being made of a nitride compound represented by AlyGa1xe2x88x92yxe2x88x92zInzN (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), the second thermal expansion coefficient T2 being lower than the first thermal expansion coefficient T1, the third thermal expansion coefficient T3 being lower than the first thermal expansion coefficient T1 and higher than the second thermal expansion coefficient T2.
In the first method of fabricating a semiconductor device, the strain reducing layer having the second thermal expansion coefficient T2 lower than the first thermal expansion coefficient T1 is formed on the substrate having the first thermal expansion coefficient T1 and then the nitride compound semiconductor layer having the third thermal expansion coefficient T3 lower than the first thermal expansion coefficient T1 and higher than the second thermal expansion coefficient T2 is formed on the strain reducing layer. Accordingly, when the temperature of the substrate is lowered from a temperature for the crystal growth of the nitride compound semiconductor layer to a room temperature after the formation of the nitride compound semiconductor layer, a compression strain applied from the substrate to the strain reducing layer and a tensile strain applied from the strain reducing layer to the nitride compound semiconductor layer cancel out each other. In other words, since the nitride compound semiconductor layer having the third thermal expansion coefficient T3 is formed on a multilayer structure composed of the substrate having the first thermal expansion coefficient T1 higher than the third thermal expansion coefficient T3 and the strain reducing layer having the second thermal expansion coefficient T2 lower than the third thermal expansion coefficient T3, the difference between a mean value of the thermal expansion coefficients of the multilayer structure and the third thermal expansion coefficient T3 can be made smaller than the difference between the first and third thermal expansion coefficients T1 and T3. As a result, an internal stress produced in the nitride compound semiconductor layer in response to the compression strain from the substrate is reduced and cracks are less likely to be formed in the nitride semiconductor layer, so that the crystalline characteristics of the nitride compound semiconductor layer are improved.
In the first method of fabricating a semiconductor device, the step of forming the strain reducing layer preferably includes the step of forming, when a ratio T1/T3 of the first thermal expansion coefficient T1 to the third thermal expansion coefficient T3 is lower than a ratio T3/T2 of the third thermal expansion coefficient T3 to the second thermal expansion coefficient T2, the strain reducing layer such that the strain reducing layer is smaller in thickness than the substrate and forming, when the ratio T1/T3 of the first thermal expansion coefficient T1 to the third thermal expansion coefficient T3 is higher than the ratio T3/T2 of the third thermal expansion coefficient T3 to the second thermal expansion coefficient T2, the strain reducing layer such that the strain reducing layer is larger in thickness than the substrate.
The arrangement further reduces the difference between the mean value of the thermal expansion coefficients of the multilayer structure composed of the substrate and the strain reducing layer and the third thermal expansion coefficient T3, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
In the first method of fabricating a semiconductor device, the substrate is preferably made of sapphire and the strain reducing layer is preferably made of silicon.
In the arrangement, the strain reducing layer having a specified growth interface is formed on the substrate, so that the crystalline characteristics of the nitride compound semiconductor layer formed on the strain reducing layer are further improved.
In this case, the step of forming the strain reducing layer preferably includes the step of forming the strain reducing layer having a growth interface having a (111) plane on the substrate having a main surface having a (0001) plane.
In the arrangement, the nitride compound semiconductor layer having a growth interface having a (0001) plane is formed on the strain reducing layer formed to have a growth interface having a (111) plane, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
A second method of fabricating a semiconductor device according to the present invention comprises the steps of: forming a strain reducing layer having a second thermal expansion coefficient T2 on a substrate having a first thermal expansion coefficient T1; and forming a semiconductor layer having a third thermal expansion coefficient T3 on the strain reducing layer, the semiconductor layer being made of a nitride compound represented by AlyGa1xe2x88x92yxe2x88x92zInzN (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), the second thermal expansion coefficient T2 being higher than the first thermal expansion coefficient T1, the third thermal expansion coefficient T3 being higher than the first thermal expansion coefficient T1 and lower than the second thermal expansion coefficient T2.
In the second method of fabricating a semiconductor device, the strain reducing layer having the second thermal expansion coefficient T2 higher than the first thermal expansion coefficient T1 is formed on the substrate having the first thermal expansion coefficient T1 and then the nitride compound semiconductor layer having the third thermal expansion coefficient T3 higher than the first thermal expansion coefficient T1 and lower than the second thermal expansion coefficient T2 is formed on the strain reducing layer. Accordingly, when the temperature of the substrate is lowered from a temperature for the crystal growth of the nitride compound semiconductor layer to a room temperature after the formation of the nitride compound semiconductor layer, a tensile strain applied from the substrate to the strain reducing layer and a compression strain applied from the strain reducing layer to the nitride compound semiconductor layer cancel out each other. In other words, since the nitride compound semiconductor layer having the third thermal expansion coefficient T3 is formed on a multilayer structure composed of the substrate having the first thermal expansion coefficient T1 lower than the third thermal expansion coefficient T3 and the strain reducing layer having the second thermal expansion coefficient T2 higher than the third thermal expansion coefficient T3, the difference between a mean value of the thermal expansion coefficients of the multilayer structure and the third thermal expansion coefficient T3 can be made smaller than the difference between the first and third thermal expansion coefficients T1 and T3. As a result, an internal stress produced in the nitride compound semiconductor layer in response to the tensile stress from the substrate is reduced and cracks are less likely to be formed in the nitride semiconductor layer, so that the crystalline characteristics of the nitride compound semiconductor layer are improved.
In the second method of fabricating a semiconductor device, the step of forming the strain reducing layer preferably includes the step of forming, when a ratio T3/T1 of the third thermal expansion coefficient T3 to the first thermal expansion coefficient T1 is lower than a ratio T2/T3 of the second thermal expansion coefficient T2 to the third thermal expansion coefficient T3, the strain reducing layer such that the strain reducing layer is smaller in thickness than the substrate and forming, when the ratio T3/T1 of the third thermal expansion coefficient T3 to the first thermal expansion coefficient T1 is higher than the ratio T2/T3 of the second thermal expansion coefficient T2 to the third thermal expansion coefficient T3, the strain reducing layer such that the strain reducing layer is larger in thickness than the substrate.
The arrangement further reduces the difference between the mean value of the thermal expansion coefficients of the multilayer structure composed of the substrate and the strain reducing layer and the third thermal expansion coefficient T3, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
A third method of fabricating a semiconductor device according to the present invention comprises the steps of: forming a strain reducing layer having a second thermal expansion coefficient T2 on a substrate having a first thermal expansion coefficient T1; forming a semiconductor layer having a third thermal expansion coefficient T3 on the strain reducing layer, the semiconductor layer being made of a nitride compound represented by AlyGa1xe2x88x92yxe2x88x92zInzN (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61); and separating the nitride compound semiconductor layer from a multilayer structure composed of the substrate and the strain reducing layer to form a semiconductor substrate composed of the nitride compound semiconductor layer, the second thermal expansion coefficient T2 being lower than the first thermal expansion coefficient T1, the third thermal expansion coefficient T3 being lower than the first thermal expansion coefficient T1 and higher than the second thermal expansion coefficient T2.
In the third method of fabricating a semiconductor device, the strain reducing layer having the second thermal expansion coefficient T2 lower than the first thermal expansion coefficient T1 is formed on the substrate having the first thermal expansion coefficient T1 and then the nitride compound semiconductor layer having the third thermal expansion coefficient T3 lower than the first thermal expansion coefficient T1 and higher than the second thermal expansion coefficient T2 is formed on the strain reducing layer. Accordingly, when the temperature of the substrate is lowered from a temperature for the crystal growth of the nitride compound semiconductor layer to a room temperature after the formation of the nitride compound semiconductor layer, a compression strain applied from the substrate to the strain reducing layer and a tensile strain applied from the strain reducing layer to the nitride compound semiconductor layer cancel out each other. In other words, since the nitride compound semiconductor layer having the third thermal expansion coefficient T3 is formed on a multilayer structure composed of the substrate having the first thermal expansion coefficient T1 higher than the third thermal expansion coefficient T3 and the strain reducing layer having the second thermal expansion coefficient T2 lower than the third thermal expansion coefficient T3, the difference between a mean value of the thermal expansion coefficients of the multilayer structure and the third thermal expansion coefficient T3 can be made smaller than the difference between the first and third thermal expansion coefficients T1 and T3. As a result, an internal stress produced in the nitride compound semiconductor layer in response to the compression strain from the substrate is reduced and cracks are less likely to be formed in the nitride semiconductor layer, so that the crystalline characteristics of the nitride compound semiconductor layer are improved. By separating the nitride compound semiconductor layer from the multilayer structure composed of the substrate and the strain reducing layer, therefore, the semiconductor substrate composed of the nitride semiconductor layer with excellent crystalline characteristics can be formed.
In the third method of fabricating a semiconductor device, the step of forming the strain reducing layer preferably includes the step of forming, when a ratio T1/T3 of the first thermal expansion coefficient T1 to the third thermal expansion coefficient T3 is lower than a ratio T3/T2 of the third thermal expansion coefficient T3 to the second thermal expansion coefficient T2, the strain reducing layer such that the strain reducing layer is smaller in thickness than the substrate and forming, when the ratio T1/T3 of the first thermal expansion coefficient T1 to the third thermal expansion coefficient T3 is higher than the ratio T3/T2 of the third thermal expansion coefficient T3 to the second thermal expansion coefficient T2, the strain reducing layer such that the strain reducing layer is larger in thickness than the substrate.
The arrangement further reduces the difference between the mean value of the thermal expansion coefficients of the multilayer structure composed of the substrate and the strain reducing layer and the third thermal expansion coefficient T3, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
In the third method of fabricating a semiconductor device, the substrate is preferably made of sapphire and the strain reducing layer is preferably made of silicon.
In the arrangement, the strain reducing layer having a specified growth interface is formed on the substrate, so that the crystalline characteristics of the nitride compound semiconductor layer formed on the strain reducing layer are further improved.
In this case, the step of forming the strain reducing layer preferably includes the step of forming the strain reducing layer having a growth interface having a (111) plane on the substrate having a main surface having a (0001) plane.
In the arrangement, the nitride compound semiconductor layer having a growth interface having a (0001) plane is formed on the strain reducing layer formed to have a growth interface having a (111) plane, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.
In this case, the step of forming the semiconductor substrate preferably includes the step of removing the strain reducing layer by using a solution containing a hydrofluoric acid.
The arrangement allows the nitride compound semiconductor layer to be separated from the multilayer structure composed of the substrate and the strain reducing layer without removing the substrate by polishing. Accordingly, the semiconductor substrate composed of the nitride compound semiconductor layer can be formed easily in a short period of time and the substrate separated from the nitride compound semiconductor layer can be used again to newly form a nitride compound semiconductor layer.
A fourth method of fabricating a semiconductor device according to the present invention comprises the steps of: forming a strain reducing layer having a second thermal expansion coefficient T2 on a substrate having a first thermal expansion coefficient T1; forming a semiconductor layer having a third thermal expansion coefficient T3 on the strain reducing layer, the semiconductor layer being made of a nitride compound represented by AlyGa1xe2x88x92yxe2x88x92zInzN (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61); and separating the nitride compound semiconductor layer from a multilayer structure composed of the substrate and the strain reducing layer to form a semiconductor substrate composed of the nitride compound semiconductor layer, the second thermal expansion coefficient T2 being higher than the first thermal expansion coefficient T1, the third thermal expansion coefficient T3 being higher than the first thermal expansion coefficient T1 and lower than the second thermal expansion coefficient T2.
In the fourth method of fabricating a semiconductor device, the strain reducing layer having the second thermal expansion coefficient T2 higher than the first thermal expansion coefficient T1 is formed on the substrate having the first thermal expansion coefficient T1 and then the nitride compound semiconductor layer having the third thermal expansion coefficient T3 higher than the first thermal expansion coefficient T1 and lower than the second thermal expansion coefficient T2 is formed on the strain reducing layer. Accordingly, when the temperature of the substrate is lowered from a temperature for the crystal growth of the nitride compound semiconductor layer to a room temperature after the formation of the nitride compound semiconductor layer, a tensile strain applied from the substrate to the strain reducing layer and a compression strain applied from the strain reducing layer to the nitride compound semiconductor layer cancel out each other. In other words, since the nitride compound semiconductor layer having the third thermal expansion coefficient T3 is formed on a multilayer structure composed of the substrate having the first thermal expansion coefficient T1 lower than the third thermal expansion coefficient T3 and the strain reducing layer having the second thermal expansion coefficient T2 higher than the third thermal expansion coefficient T3, the difference between a mean value of the thermal expansion coefficients of the multilayer structure and the third thermal expansion coefficient T3 can be made smaller than the difference between the first and third thermal expansion coefficients T1 and T3. As a result, an internal stress produced in the nitride compound semiconductor layer in response to the tensile stress from the substrate is reduced and cracks are less likely to be formed in the nitride semiconductor layer, so that the crystalline characteristics of the nitride compound semiconductor layer are improved. By separating the nitride compound semiconductor layer from the multilayer structure composed of the substrate and the strain reducing layer, therefore, the semiconductor substrate composed of the nitride semiconductor layer with excellent crystalline characteristics can be formed.
In the fourth method of fabricating a semiconductor device, the step of forming the strain reducing layer preferably includes the step of forming, when a ratio T3/T1 of the third thermal expansion coefficient T3 to the first thermal expansion coefficient T1 is lower than a ratio T2/T3 of the second thermal expansion coefficient T2 to the third thermal expansion coefficient T3, the strain reducing layer such that the strain reducing layer is smaller in thickness than the substrate and forming, when the ratio T3/T1 of the third thermal expansion coefficient T3 to the first thermal expansion coefficient T1 is higher than the ratio T2/T3 of the second thermal expansion coefficient T2 to the third thermal expansion coefficient T3, the strain reducing layer such that the strain reducing layer is larger in thickness than the substrate.
The arrangement further reduces the difference between the mean value of the thermal expansion coefficients of the multilayer structure composed of the substrate and the strain reducing layer and the third thermal expansion coefficient T3, so that the crystalline characteristics of the nitride compound semiconductor layer are further improved.